The present invention relates to charge transporting materials and electrophotographic photoconductors using the same, and more particularly, novel aminobiphenyl compounds used as the charge transporting materials, and electrophotographic photoconductors comprising any of the aminobiphenyl compounds as an organic photoconductive material or charge transporting material.
Conventionally a variety of inorganic and organic electrophotographic photoconductors are known. As inorganic photoconductors for use in electrophotography, there are known types in which the photoconductive materials, for instance, selenium, cadmium sulfide, and zinc oxide.
In an electrophotographic process, a photoconductor is first exposed to corona charges in the dark, so that the surface of the photoconductor is electrically charged uniformly to a predetermined polarity. The thus uniformly charged photoconductor is then exposed to original light images and the portions exposed to the original light images selectively become electroconductive so that electric charges dissipate from the exposed portions of the photoconductor, whereby latent electrostatic images corresponding to the original light images are formed on the surface of the photoconductor. The latent electrostatic images are then developed by the so-called toner which comprises a colorant, such as a dye or a pigment, and a binder agent made, for instance, of a polymeric material; thus, visible developed images can be obtained on the photoconductor. It is necessary that the photoconductors for use in electrophotography have at least the following fundamental properties: (1) chargeability to a predetermined potential in the dark; (2) minimum electric charge dissipation in the dark; and (3) quick dissipation of electric charges upon exposure to light.
While the above-mentioned inorganic electrophotographic photoconductors have many advantages over other conventional electrophotographic photoconductors, at the same time they have several shortcomings from the viewpoint of practical use.
For instance, a selenium photoconductor, which is widely used at present and sufficiently meets the above-mentioned requirements (1) to (3), has the shortcoming that its production is difficult and, accordingly, its production cost is high. Further, it is difficult to work it into the form of a belt due to its poor flexibility, and it is so vulnerable to heat and mechanical shocks that it must be handled with the utmost care.
Cadmium sulfide photoconductors and zinc oxide photoconductors are prepared by dispersing cadmium sulfide or zinc oxide in a binder resin. They can be produced inexpensively compared with selenium photoconductors and are also used commonly in practice. However, the cadmium sulfide and zinc oxide photoconductors are poor in surface smoothness, hardness, tensile strength and wear resistance. Therefore, they are not suitable as photoconductors for use in plain paper copiers in which the photoconductors are used in quick repetition.
Recently, organic electrophotographic photoconductors, which are said not to have such shortcomings of the inorganic electrophotographic photoconductors, have been proposed, and some of them are in fact used in practice.
Representative examples of such organic electrophotographic photoconductors are an electrophotographic photoconductor comprising poly-N-vinylcarbazole and 2,4,7-tri-nitrofluorenon-9-one (U.S. Pat. No. 3,484,237), an electrophotographic photoconductor comprising poly-N-vinylcarbazole which is sensitized by a pyrylium salt type dye (Japanese Patent Publication 48-25658), photoconductors containing as the main component organic pigments (Japanese Laid-Open Patent Application 47-37543), a photoconductor comprising as the main component an eutectic crystalline complex consisting of a dye and a resin (Japanese Laid-Open Patent Application 47-10735), a photoconductor comprising a triphenylamine compound which is sensitized by a dye (U.S. Pat. No. 3,180,730), and a photoconductor comprising as charge transporting materials poly-N-vinyl-carbazole and an amide derivative/Japanese Laid-Open Patent Application 58-1155).
Although the above-mentioned organic electrophotographic photoconductors have various advantages over other conventional electrophotographic photoconductors, they do not meet various requirements in view of the practical use thereof.
Furthermore, polyfunctional tertiary amine compounds, in particular, aminobiphenyl derivatives (or benzidine derivatives), are known as excellent photoconductive materials for use in electrophotographic photoconductors, as disclosed in U.S. Pat. No. 3,265,496, Japanese Patent Publication 39-11546, and Japanese Laid-Open Patent Application 53-27033. These compounds, however, have the shortcoming that they are so slightly soluble in adhesive resins that they tend to be crystallized in a photoconductive layer of an electrophotographic photoconductor. In order to eliminate this shortcoming, it is tried to use the above compounds in combination with low-molecular weight compounds for minimizing the crystallization of the amine compounds as disclosed in Japanese Laid-Open Patent Application 62-11216.
In addition to the above aminobiphenyl compounds, N,N-diphenyl-[1,1-biphenyl]-4-amine (Helv. Chim. Acta. 6 1011P, 1923), N,N-bis(4-methoxyphenyl)-[1,1'-biphenyl]-4-amine, N,N-bis([1,1'-biphenyl]-4-yl)-[1,1'-biphenyl]-4-amine and N-[1,1'-biphenyl]-4-yl-N-(4-methoxyphenyl)-[1,1'-biphenyl]-4-amine (J. Prakt. Chem. 317 (2) 284P, 1975) are known. These compounds, however, are not useful as organic photoconductive materials for electrophotography.
Furthermore, Japanese Laid-Open Patent Application 57-195254 discloses N,N-diphenyl[1,1'-biphenyl]-4-amine as a carrier transporting material for use in an electrophotographic photoconductor. However, the electrophotographic photoconductor using the above compound has the shortcomings that the residual potential thereof is built up while in repeated use thereof and accordingly the obtained image quality is degraded while in use, with the production of unclear images.
Japanese Laid-Open Patent Application 62-201447 discloses carrier transporting materials such as 4-dimethylamino-4'-diphenylaminobiphenyl and 4-diethylamino-4'-di(m-tolylamino)-biphenyl. The electrophotographic photoconductors using these compounds, however, have the shortcomings that the chargeability thereof decreases while in repeated use and accordingly the obtained images become unclear.